Chlorine dioxide is known to be produced continuously in a generator vessel by reduction of sodium chlorate in the presence of sulphuric acid in an aqueous reaction medium maintained at its boiling point under a subatmospheric pressure and from which by-product sodium sulphate deposits. The chlorine dioxide is removed from the generator in gaseous admixture with the stream.
A slurry of the deposited sodium sulphate and spent reaction medium is removed from the generator, the solid sodium sulphate is filtered out and the spent reaction medium is recycled to the generator, after the addition of make-up reactants.
The recycle line usually communicates with a sodium chlorate feed line upstream of a reheater, in which the recycle stream is reheated to the reaction temperature. As is described in U.S. Pat. No. 3,895,100, assigned to the assignee herein, the disclosure of which is incorporated herein by reference, a venturi pipe is included in the recycle line, the back pressure from which prevents boiling of recycle liquor in the reheater. Concentrated sulphuric acid is added to the throat of the venturi to be mixed with the recycle liquor. The mixture then is permitted to expand and enter the generator vessel.
All chlorate-based chlorine dioxide generating processes produce chlorine dioxide by the reaction: EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O (1)
with a competing reaction: EQU ClO.sub.3.sup.- +5Cl.sup.- +6H.sup.+ .fwdarw.3Cl.sub.2 +3H.sub.2 O (2)
The extent to which equation (1) predominates over equation (2), i.e. the extent to which 1 mole of chlorate ion forms chlorine dioxide, is the efficiency of the process. The chloride ions employed in these reactions may be introduced from an external source and/or produced in situ by reduction of the by-product chlorine.
One commercial operation for the production of chlorine dioxide using a process of the above-described type is the so-called "R3" process developed by the assignee hereof, in which the chloride ions are provided by adding the same to the generator. One convenient measure of the efficiency for this type of chlorine dioxide-generating process, i.e. in which chloride ion reductant is fed from an external source, is the so-called Gram-Atom Percent or GA % efficiency. This measure of efficiency is based on a determination of the number of grams-atoms of chlorine in chlorine dioxide and in chlorine in the off-gas stream from the generator. The efficiency is determined by the relationship: ##EQU1## As will be seen from the above equation (1), the GA % ClO.sub.2 efficiency cannot exceed 50%.
It has been observed by the applicants for many years that the GA % efficiency of chlorine dioxide production in an R3 commercial plant was always less on a plant scale than could be attained in the laboratory. In addition, the chemical efficiency, i.e. the extent to which sodium chlorate reacts to form chlorine dioxide, was less than was expected, leading to higher-than-expected usage of sodium chlorate.
It has now surprisingly been found that the source of this problem is the decomposition of formed chlorine dioxide, mainly at the point of concentrated sulphuric acid addition. A number of subjective observations have been made by the assignee over the last 15 years or more of commercial operation of the R3 process which have led to this conclusion. In the early days of engineering the R3 process, a safe parameter for the number of concentrated sulphuric acid feed nozzles for the design capacity of chlorine dioxide generators was established. This parameter was arived at based on the observation that at greater tons/day capacity per acid feed nozzle, crackling could be heard coming from the venturi, which was thought to be the result of chlorine dioxide decomposition. As the demand for larger production capacity generators grew, the requirement for the number of concentrated sulphuric acid entry nozzles based on the traditional formula became impractically large to pipe.
The original generator design had a radial entry for the recycle line (see U.S. Pat. No. 3,895,100 mentioned above). However, more modern generators use a tangential entry of the recycle line (see U.S. Pat. No. 4,203,961, assigned to the assignee herein), which permits increased production rates. However, as production rates increased, a decrease in efficiency was obtained. Direct observation of the vortex formed in the generator detected the incidence of "puffing" or spontaneous decomposition of the chlorine dioxide. The installation of vortex breakers in the slurry outlet from the generator solved this problem and the efficiency was restored to higher levels, although still below the expected level based on laboratory experience.
The observation of actual chlorine dioxide decompositions in the generator has lead to the conclusion that the less than expected efficiency in the plant results from decomposition of chlorine dioxide at the inlet of concentrated acid to the venturi. Concentrated sulphuric acid has a considerable heat of dilution, which may result in the development of local "hot spots", causing local decomposition of chlorine dioxide by the equation: EQU ClO.sub.2 .fwdarw.1/2Cl.sub.2 +O.sub.2
An extreme example of such decomposition is the crackling observed above, where the decompositions have become so numerous that they become audible.
The formation of such local "hot spots" results from incomplete and non-uniform mixing of the introduced acid and the recycled reactants. Once the source of this more than fifteen year old problem was recognized, the applicants sought solutions to the problem. If indeed the problem lay in incomplete mixing of the concentrated sulphuric acid, predilution of the acid should provide the sulphuric acid in a more compatible and dispersible form without a tendency for "hot spot" formation and decreased efficiency. Indeed, when 63% H.sub.2 SO.sub.4 sulphuric acid was substituted for the concentrated (93 to 95 wt. % H.sub.2 SO.sub.4) sulphuric acid, the efficiency of chlorine dioxide production increased to close to expected levels.
However, diluting the sulphuric acid in this way increases significantly the evaporative load on the generator and hence the energy requirements for the same production rate of chlorine dioxide and was considered undesirable.